1. Field of the Invention
The present invention relates to an optical pickup for generating a plurality of light beams having maximally condensed points different from each other to cope with various types of discs, which distances from disc surfaces to information record surfaces thereof (i.e., the thicknesses of the protection layers of the discs) are different from each other, such as a CD (Compact Disc), a DVD (i.e., a high recording density disc which has the same size as the CD and which record capacity is much larger than the CD) and the like by use of a single information reproducing apparatus.
2. Description of the Related Art
There is a DVD as a high recording density disc, which recording capacity is much larger than the conventional CD and is capable of recording one movie etc., and there is being developed a CD/DVD compatible player which is capable of reproducing both of the DVD and the CD.
Comparing the structures of the CD and the DVD with each other, the thickness of the protection layer of the DVD (about 0.6 mm) is about half of that of the CD (about 1.2 mm) because of the requirement of improving the recording density. Therefore, if both of the discs are to be reproduced by use of a single optical pickup having just one focal point, e.g., if the optical pickup converges the light so as optimize the light beam for the DVD, the aberration such as a spherical aberration etc., that is generated is severe for the CD, since the protection layer of the CD through which the light beam passes is thicker than that of the DVD, resulting in such a problem that the optical pickup cannot converge the light beam to be optimal for the CD.
Further, the sizes of information pits formed to record the information are different from each other between the CD and the DVD (more concretely, the length of the shortest pit among the information pits formed on the CD is about 0.87 .mu.m while that on the DVD is about 0.4 .mu.m). Thus, in order to precisely read the respective information pits on these two types of discs, it is necessary to form the light spot optimum for the size of the information pit on each of the CD and the DVD.
The size (i.e. the diameter) of the light spot is proportional to the ratio of the wavelength of the light beam and the numerical aperture of the objective lens to converge the light beam onto the information record surface. Namely, assuming that the wavelength of the light beam is constant, as the numerical aperture is increased, the size of the light spot is increased. Thus, in case that the CD and the DVD are reproduced by the optical pickup having one focal point, if the numerical aperture is set optimum for the information pit on the DVD while the wavelength of the light beam is set constant, there is raised such a problem that the light spot is too small for the information pit on the CD, so that the distortion is generated in the reproduction signal when reproducing the CD and that the information cannot be precisely read.
Therefore, in order to solve the above mentioned problems, there is a bifocal optical pickup having a bifocal lens capable of emitting two light beams, which have maximally condensed points at positions different from each other on a same straight line and which form two light spots respectively optimum for the sizes of the information pits on the CD and the DVD.
The bifocal lens is constructed by arranging a diffraction grating such as a hologram diffraction grating or the like on a optical path same as that of the objective lens, so as to separate an incident light beam, which has been collimated by a collimator lens, into three light beams i.e., a 0 order light, a +1 order light and a -1 order light by the diffraction grating. By utilizing the fact that the hypothetical position of a light emitting point of the 0 order light and that of the +1 order light after the separation (which are the positions of the light emitting point of the 0 order light and the +1 order light after the separation in case of viewing them from the DVD side or the CD side of the objective lens. Since the path of the 0 order light and the path of the +1 order light are different from each other, it can be hypothetically supposed that the light emitting points of these two lights which are based on the paths after the separation are different from each other) are different from each other with respect to the objective lens, the 0 order light and the +1 order light are converged at different positions on a same straight line. At this time, among the light beams, the light beam passes through the diffraction grating becomes the 0 order light, and the light beam diffracted by the diffraction pattern of the diffraction grating becomes the +1 order light.
More concretely, as for the positions of the focal points, the bifocal lens is constructed such that the +1 order light is focused at a position farther from the objective lens than the 0 order light, and that the 0 order light is converged optimally for the information record surface of the DVD, while the +1 order light is converged optimally for the information record surface of the CD.
By use of the bifocal lens, it is possible to reproduce both of the CD and the DVD by use of a single optical pickup.
However, in the above explained bifocal optical pickup, since the light beam is separated by the diffraction grating, there are generated the -1 order light and some higher order lights other than the 0 order light and the +1 order light which are to be emitted onto the DVD and the CD respectively. Further, in case of reproducing the DVD, the +1 order light becomes one of the useless lights, while the 0 order light becomes one of the useless lights in case of reproducing the CD. Then, there may happen such a case that these useless lights are returned to a light detector, which are to detect the reflection light from the CD or the DVD, as a so-called stray light. When this case happens, the detection output due to the stray light is mixed as a noise into the detection signal to be primarily detected, so that an accurate information reproduction cannot be performed, which is a problem.